1. Field of the Invention
The present invention relates to a data delivery system, and more particularly, to a data delivery system suitable for use, for example, by a movie distribution company to deliver movie data to a movie theater via a relay server.
2. Description of the Related Art
In recent years, a practical technique has been developed for transmitting a video source (video content data), for example, on a video-on-demand basis. This makes it possible to supply various types of video sources without having to use a film or a video cassette tape.
A conventional movie distribution system is described below with reference to FIG. 56.
Usually, a movie produced by a movie production company 500 is distributed, via movie distribution companies 501a, 501b, 501c, and so on, to movie theaters 502a, 502b, 502c, 502d, and so on under contract.
After shooting a movie, a movie producer edits a negative master film by cutting and connecting the film. After completion of the editing, an edited positive film is produced from the edited negative master film. The movie production company 500 produces a negative master film from which films to be distributed will be produced.
From this edited negative master film, a desired number of edited positive films are produced and delivered (transported) to movie distribution companies 501a, 501b, and so on.
Each of movie distribution companies 501a, 501b, and so on produces a large number of negative films from a delivered positive film. The produced negative films are delivered (transported) from the movie distribution companies 501a, 501b, and so on to movie theaters 502a, 502b, and so on in accordance with a contract. The movie theaters 502a, 502b, and so on show the movie using distributed films.
If the playing period predetermined in accordance with the contract between the respective movie theaters 502a, 502b, and so on and a movie production company 500 or a movie distribution company 501 has expired, the negative films are returned to the movie distribution company 501a from movie theaters 502a, 502b, . . . , and so on.
The movie distribution companies 502a, 502b, and so on dispose the negative films returned from the movie theaters thereby preventing the films from being used in an unauthorized manner.
Alternatively, the movie production company 500 collects all movie films from the movie distribution companies 502a, 502b, and so on to prevent unauthorized use of the films.
In the conventional movie distribution system, distribution of a large number of films among movie production companies 500, movie distribution companies 501, and movie theaters 502 and production of a large number of films to be supplied to movie theaters 502 need very high cost and very long time.
In view of the advance in the technology of data delivery system, it will be advantageous to deliver a movie not via a film but via electronic data. This can reduce the delivery cost and the delivery time.
However, to realize a movie distribution system by means of electronic data, the following problems should be resolved.
It is required to protect the copyright of movie content data. To meet this requirement, it is needed to surely prevent distributed content data from being copied in an unauthorized manner. In particular, this requirement becomes very important when a video source is transmitted in the form of electronic data which can be more easily copied than a film.
For example, there is a possibility that content data is stolen from a middle of a transmission line via which the content data is transmitted.
Unlike movie films, content data is not corporeal. This can cause content data to be easily copied at a movie theater site without permission. This means that it is impossible to completely prevent an unauthorized copy of content data by means of withdrawing, unlike films.
A possible technique of preventing distributed video contents from being copied without permission is to encrypt video content data. However, in this case, it is required to transmit key data used by movie distribution companies 501 or movie theaters 502 to decrypt the encrypted video content data. When a decryption key data is transmitted in a similar manner to content data, if the key data is hacked together with the video content data during transmission process, an unauthorized copy can be made.